1. Technical Field
This invention relates to image processing techniques and, more particularly, to a method and apparatus for thresholding analog video signals.
2. Discussion
One of the major problems in performing efficient digital processing of an image is to properly digitize the analog video signal from the detector or source. Traditionally, the amplitude or intensity of the analog video signal from the detector is compared against a fixed threshold. Analog levels above the threshold are converted to a binary or digital one whereas analog signal levels below the fixed threshold are converted to a digital zero level.
The chosen thresholding technique should allow detection of the target signal in the various kinds of noise that are possible. There are two primary kinds of noise. One is low-frequency shading or scan noise that can be likened to a parabola along the scan direction. The other kind of noise is high frequency noise generated by the detector itself. As shown in FIG. 1, a fixed thresholding technique creates false detections that are due to noise instead of actual scene content.
In selecting criteria on which to base a threshold level, a designer is usually trapped between conflicting limitations. If the threshold is weighted too heavily toward the background (a "low" threshold), excessive digitization or clutter may occur. On the other hand, if the threshold is set too "high", a loss of digitized target pixels may result. The use of a low-pass filter does not provide advantageous results because the low frequency noise component would cross the threshold, just as a high-pass filter would allow the high frequency noise component to cross the threshold. A band pass filter is not easily implementable because there is little reliable information within the target spatial frequency prior to target selection. This is due to the uncertainty in target size, range to the target or the unknown effects of image blur of the target signal.